A problem with implementing MBMS is that where there are cells on different frequency layers, with a distribution of MBMS subscribers across the different frequency layers, system capacity can be wasted if the same MBMS data stream is transmitted on the different frequency layers to UEs in the same geographical coverage area. Examples include overlapping cells, whether co-located or not. The term “co-location” in general is understood as cells on different frequencies have the same base station site i.e. the transmission of two or more carrier frequencies happens from the same base station site. However, in practice the network may have two or more carrier frequencies, which have different cell sizes and therefore at least not all cells are co-located although the coverage of these different frequency layers are overlapping in a given geographical area. Hence, the problem does not only occur in co-located case but in all case and areas where the network has cell coverage on more than one frequency and therefore a network operator would like to provide MBMS service only on one of the carrier frequencies (or at least not on all carrier frequencies)
3GPP TS 25.346 v6.0.0 discusses general structural and functional aspects of providing MBMS in a RAN. Currently 25.346 v.6.0.0 defines frequency layer convergence as follows:
“Frequency Layer Convergence denotes the process where the UTRAN requests UEs to preferentially re-select to the frequency layer on which the MBMS service is intended to be transmitted. This layer preference could be done by an additional MBMS session related Layer Convergence Information (LCI) such as offset and target frequency. These kinds of information could be given to UEs at session start and during the whole session, and will be applied during the entire session. More than one offset may be required to support multiple frequencies, but it is assumed that the same LCI information will apply to all the services on the same frequencies.”
R2-031716, 3GPP TSG RAN WG2 meeting #37 Budapest, Hungary, Aug. 25-29, 2003 proposes frequency layer convergence (FLC) for MBMS.
R2-032077, TSG RAN WG2 meeting #38 Sophia Antipolis, France, Sep. 6-10, 2003 discloses a UE layer convergence mechanism for MBMS.
R2-040086, TSG RAN WG2 meeting #38 Sophia Antipolis, France, Sep. 6-10, 2003 discusses different alternatives for layer convergence and possible MBMS frequency layer convergence procedures.
3GPP TS 25.346 v6.0.0 discusses general structural and functional aspects of providing MBMS in a RAN. Currently 25.346 v.6.0.0 defines frequency layer convergence as follows:
“Frequency Layer Convergence denotes the process where the UTRAN requests UEs to preferentially re-select to the frequency layer on which the MBMS service is intended to be transmitted. This layer preference could be done by an additional MBMS session related Layer Convergence Information (LCI) such as offset and target frequency. These kinds of information could be given to UEs at session start and during the whole session, and will be applied during the entire session. More than one offset may be required to support multiple frequencies, but it is assumed that the same LCI information will apply to all the services on the same frequencies.”
The intention behind the FLC concept is to maximise the point-to-multipoint (p-t-m) connections for an MBMS session, from the radio network controller (RNC) point of view. This is because MBMS in point-to-point (p-t-p) would not present many advantages over R99 DCH connections since the same radio resources are consumed. Thus, FLC reduces system capacity wastage in that where there is overlapping cell coverage on different frequency layers, with a distribution of MBMS subscribers across the different frequency layers, the MBMS subscribers are moved onto the same frequency layer thereby negating the need to transmit the same MBMS data stream on multiple different frequency layers to UEs in the same geographical coverage area.
The discussion in 25.346 v.6.0.0 and the proposals in documents R2-031716, R2-032077 and R3-040086 do not define the details required to implement a frequency layer convergence method for MBMS but instead discuss generally the requirements for frequency layer convergence.
It is currently not possible to converge a UE to a certain cell or frequency, which is different from the one that would be selected or reselected based on the normal cell reselection criteria in the following radio resource control (RRC) states: Idle, CELL_FACH, CELL_PCH. Furthermore, it is also not possible to keep the UE on a preferred MBMS frequency layer when the cell reselection criteria indicate a cell on another carrier frequency to be better and no there is no cell barring.